Such a method is, for example, known from U.S. Pat. No. 5,644,458. Here, stator voltages, stator currents and stator fluxes are transformed from a 3-phase into the stator based 2-phase coordinate system. Subsequently, an additional transformation into the rotor based coordinate system with reference to the rotor flux. In a motor model, these transformed values are further linked with fixed parameters. The current components precalculated in this model are compared to the real current components. When deviations occur, which are larger than a certain percentage in the range from 1% to 25%, an unspecified fault in the motor is assumed.
U.S. Pat. No. 6,822,839 B2 also describes the commonly known coordinate transformation of a 3-phase induction system into a two-axis system. An additional transformation is applied to get information about the real power and the reactive power. By means of the total reactive motor power, a fault detection is performed. However, with a loaded motor this has only minor significance.